8/8/2019 Dimmable Electronic Ballast With Unity Power Factor Based on a Single-Stage Current-Fed Resonant Inverter

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Dimmable Electronic Ballast with Unity

Power Factor Based on a Single-StageCurrent-Fed Resonant Inverter

CONVENTIONAL TOPOLOGY:

In fluorescent lamps, it is well known that a ballast is required

to provide sufficient high voltage for proper lamp ignition and

stabilize the lamp current once the lamp arc is established. The

conventional line-frequency magnetic ballast is heavy and

bulky, which makes the device itself too large to be installed

inside the CFL.

The low operating frequency also causes light flickering, and it

is impossible to implement dimming operations with a

magnetic ballast. As a result, electronic ballast becomes the

most suitable candidate to be used in CFL applications.

A typical electronic ballast configuration consisting of a power

factor correction (PFC) stage and a resonant inverter. This two-

stage circuit results in a high-cost and large-size circuit. Single-

stage inverters (SSIs) are then proposed by combining the PFC

switch and one of the switches in the half-bridge resonant

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inverter together using the synchronous switch concept. This

design presents significant improvements in size and cost

reduction over the conventional two-stage circuit. The main

drawback with the SSI is the need of a complicated control

circuit to function as dc-link voltage regulation and output-lamp

current regulation.

PROPOSED TOPOLOGY:

In this project, a single-stage dimming electronic ballast with variable

frequency control using an integrated buck-boost PFC current-fed

resonant inverter is proposed. Although the efficiency of the SSI tends

to be lower than the conventional design approach, the proposed SSI

circuit saves one MOSFET and a PFC controller compared to the two-

stage high-power-factor ballast circuit.

It also allows a simple-switch driver circuit to be used without any

isolation devices. The controller circuit utilizes the use of a simple

lamp dimmer that is designed using a modified 555 timer to achieve

dimming operation by varying the switching frequency of the resonant

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circuit. By closed-loop feeding the lamp current to the proposed

controller, the lamp is always driven by a current source, and stability

can be guaranteed.

Abstract

A single-stage dimmable electronic ballast with veryhigh power factor and

high efficiency is presented in this project. A compact lamp power circuit is

designed by integrating a buck-boost power factor corrector with a current-

fed resonant inverter. This integration produces a single power-processing

unit that minimizes the number of circuit components. The proposed

resonant inverter reduces the circulating current in the resonant tank and

allows simple gate drivers to be used so that isolation devices can be

eliminated. With the buck-boost stage operating in the discontinuous

conduction mode (DCM), a close-to-unity power factor is achieved for

different lamp-power conditions. A small-signal analysis is presented, and a

simple variable frequency controller is developed using the modified 555

timer to provide dimming operations.

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ADVANTAGES OF THE PROPOSED CONVERTER:

The proposed single-stage electronic ballast saves a controller for the

PFC stage and allows simple gate drivers without isolation.

High efficiency

High power factor.

APPLICATIONS OF PROPOSED TOPLOGY:

1. Lighting applications.